beds a of limestone were exposed to watery action, and broken up in part, so that fragments and pebbles of
them are found collected into beds among the mass of
later level sandy deposits b (Mendip Hills): and in
fig. 2. the same inclined limestone beds a are covered by horizontal oolitic strata c; and are worn and polished on the face of junction, and penetrated into holes by boring shells, which lived in the oolitiferous sea, long after the elevation of the older rocks. (1830.)
Length of the Scale of Time.
The scale of geological time given by the series of stratified rocks is one of unequal parts: for it is almost certain that the deposition of a given thickness of sandstone, was accomplished in a different time from that consumed in the production of an equal thickness of clay, limestone, coal, &c. Yet as many of the groups of strata contain both, sandy, argillaceous, and calcareous members, there is less error in estimating the re-
